Scissors lift mechanisms in general are known to the art. The principal purpose of such mechanisms is to provide a safe and efficient means for supporting a working platform at any desired elevation. The scissors lift mechanisms of the prior art are predicated on the well-known "lazy tong" principle, and each comprises a pair of vertically extensible scissors linkages mounted on a frame in laterally spaced, parallel relationship, and a working platform mounted on top of the linkages.
Each of the scissors linkages of the prior art lift mechanisms comprise pairs of arms pivotally connected to one another at their ends and at their centers. The lowermost pairs of arms of the linkages are pivotally mounted at one end to the frame, and they are slidably mounted on the frame at their other end. It is usual in the prior art scissors lift mechanisms to provide an hydraulic drive cylinder mechanism which is pivotally mounted to the frame, and which is coupled to a cross-bar extending between the lowermost pairs of arms of the linkages. The hydraulic lift mechanism serves to turn the arms of the lowermost pair about their pivotal axis to extend or retract the linkages and thereby to raise or lower the platform.
A disadvantage in the prior art hydraulic drive is the fact that as the lift mechanism is initially elevated from its lowermost position, the hydraulic cylinder/piston unit of the prior art hydraulic mechanism is positioned almost horizontal, and it must exert an excessively high trust on the mechanism to turn the lower-most arms and to start the vertical extension of the linkages.
Then, as the prior art lift is extended more and more in a vertical direction, the hydraulic lift unit pivots to an upright position, and it requires less and less thrust to move the load. This results in the need for an excessively large hydraulic lift unit in the prior art scissors lift in order to be effective to move the linkages from their retracted to their fully extended position, and it often leads to the requirement for auxiliary hydraulic lift mechanisms, as described below.
In addition to the above, the lift mechanisms of the prior art are designed so that the arms do not necessarily open uniformly due to the fact that the arms tend to deflect at unsupported locations thereon. In essence, when examining the pivot points connecting a pair of arms in each of a pair of transversely spaced apart scissors linkages, the pivot points in the spaced apart lowermost arms will have a variable difference with respect to the spacing between the pivot points in the uppermost arms during the initial opening. This problem results from the inelastic instability which is inherent in a beam of the type constituting an arm in a scissors linkage.
There have been many attempts to overcome the problems created by the need for excessively large hydraulic lift units, and to overcome the problems created by the inelastic instability in the scissors lift arms with attempts to employ some form of a somewhat vertically disposed hydraulic lift unit. Thus, one such attempt has been described in the U.S. Pat. No. 3,259,369 to Gridley. However, such prior art attempts have usually resulted in excessive structure in the scissors lift unit in order to support the hydraulic unit.
Moreover, in most constructions, these hydraulic units had at least one end thereof directly connected to the pivot point or to a member which was co-parallel in space with a pivot point connecting two corresponding arms of two transversely spaced apart scissors linkages. This structure tended to create some inherent instability and also required a greater amount of opening force when compared to offsetting the hydraulic drive units from the pivot points.
The improved construction of the present invention includes an hydraulic cylinder/lift unit which is mounted in an essentially fixed angular position such that the load vector is essentially aligned with the vertical axis of the unit, so that the trust exerted by the unit is essentially in the direction of the load. Moreover, the hydraulic cylinder/lift unit in the mechanism of the present invention is mounted such that the thrust exerted by this unit remains essentially invariable to move the load through all positions of the linkages. This construction results in minimizing the required capacity of the hydraulic lift unit without in any way detracting from the efficiency and safety of the unit, and thus results in a more economical lift which is capable of movement from a fully compact position to a fully extended position in a simple, economical and efficient manner by means of an hydraulic unit having a fraction of the capacity required in the prior art scissors lift. Saddle mechanisms pivotally secure the upper and lower ends of the hydraulic lift unit in such manner that these units remain in essentially vertical positions.
It is, therefore, the primary object of the present invention to provide a lift in which the hydraulic mechanism is capable of performing a desired function with less thrust and more capacity requirement than any prior art mechanism and on a more economical and safer basis.
It is another object of the present invention to provide a lift of the type stated which provides uniform load transfer to each of the arms in the scissors lift mechanism forming part of the lift.
It is a further object of the present invention to provide a lift of the type stated which overcomes the inelastic instability which otherwise results in beam deflection in prior art types of lift devices.
It is an additional object of the present invention to provide a scissors lift mechanism cooperating with hydraulic power units and which are capable of being used in a wide variety of devices.
With the above and other objects in view, my invention resides in the novel features and form, construction, arrangement and combination of parts presently described and pointed out in the claims.